So strictly speaking a peasant has a relationship to the means of production which is direct, so it's very similar to that of the petit-bourgeoisie under capitalism in that respect.

A peasant would work the land to produce most of what they need and they would trade or sell the excess to be able to access other things (think like healthcare, religious objects/texts, potentially being able to send their child to school depending on the time/place etc.)

Peasants definitely still existed in China and Russia during their revolutions and it still existed in Vietnam but to what extent I am uncertain. I know for a fact that you had Hmong in Sa Pa who were peasants (and who were very important to the Vietnamese liberation efforts) but as for the more urban areas of Vietnam idk. As for Cuba, same deal - I don't know enough to be able to say definitively.

Also note that these lines are perhaps blurrier than we might like them to be. All things that exist are inherently dialectical and it's not like a new calendar year started and suddenly all the peasant masses dropped their pitchforks and took up factory jobs; this was a gradual process and it's not out of the realms of possibility that a person may have farmed the land and then on the side worked as a prole seasonally when the land was fallow.

What this means is that it's going to be a bit more difficult on the ground to determine classes than it is to talk about them in a sort of essentialised way like Marx does; he was writing to expound his analysis of capitalism and his thesis, he wasn't developing a phylogenetic tree of classes and determining where each individual neatly fit within it.

does the peasant class still exist in the imperial core?

Actually, yes. But in the absolute margins when we're talking about western countries. There are still a few people here and there who will directly work the land and trade what they produce etc. but that's extremely rare and it exists in the tiniest way that it is not of any relevance to us today.

The people who you mention are almost certainly petit-bourgeoisie, although there are others who are bourgeois too but that's definitely one rung up the economic ladder.

this also brings up another couple questions: first, would a proletarian revolution in the imperial core today be able to rely on such a class of farmers (weather they qualify as peasant of petty bourgeoisie) to be sympathetic or does historical precedent make it seem more likely they would a reactionary opponent to progress?

It depends on the conditions. In India many farmers are extremely progressive or even radical. They're going to be one of the main groups to watch in that country, aside from the parties and the Naxalites.

In my country? Most of them are reactionary as fuck. They're going to be the worst sort of counterrevolutionaries when the revolution comes here.

second, If the proletariat cannot rely on the support of any other class, how would revolutionary strategy differ from previous revolutionary movements?

Well, the good news is that in most countries the masses have been entirely proletarianised. A century ago, give or take, in the backwards countries like Russia and China they needed the peasants to make up a large enough revolutionary base. In my country and almost certainly in yours too, the overwhelming majority of people are proles and that means that we do not need to unify with other classes to get the numbers necessary to bring about a revolution.

Note here that I'm skirting around the discussion on Mao's take on the national bourgeoisie and the Soviet NEP era because that's a massive discussion in itself and it is liable to stir up infighting. I think that at least in large part, the national bourgeoisie or NEPmen were necessary in the USSR and China because they were so under-industrialised which put them at a massive disadvantage which needed to be made up with all haste. At least in my country there are few truly national bourgeoisie and for a so-called "post industrial" society, we would need to set about on a rapid course of reindustrialisation if we were ever going to withstand the inevitable grey zone warfare that we would face as well as addressing the necessity of reparations for the countries which we have imperialised and colonised. I don't really see the role of the national bourgeoisie in this as being particularly helpful tbh.

Edit: Oh yeah, I forgot to mention that the Kulaks are a really good example of how people straddled across classes or existed in the blurry boundaries between them - so speaking strictly from the perspective of history and anthropology, people would assume the kulak... let's call that a sub-class... as they had success in their farming and accumulated enough stuff (think like livestock etc.) People tended to assume the kulak sub-class later in life and they would employ workers on their lands/livestock in order to accrue enough wealth to retire on. Towards the end of their lives, these people would end up dropping out of the kulak class as they exhausted their wealth.

So in Russia you had peasant-ish/proletarian-ish farm workers who worked for the kulaks, and these kulaks were a sort of blend between peasant and petit-bourgeoisie, or even outright bourgeoisie in the cases where they were the most successful.

Of course there were plenty of peasants who never managed to assume the sub-class of kulak due to circumstances, and of course there was generational wealth that meant some people were born into being kulaks. But it's really interesting to look into deeply.

Also note that if this arrangement continued on uninterrupted then we would naturally see an accumulation of wealth and farmland (and other important things like livestock) in the hands of fewer and fewer kulaks, with economic empires and dynasties developing through that. In the long term this would have gradually proletarianised the other peasants if it continued on its trajectory unabated imo, although in the real world due to things like famine and war often the slate was wiped clean(ish) and so perhaps due to outside factors it might have never developed to the point of proletarianising the peasants - I would find this argument to be pretty convincing too.