The irony is I think a lot of people do Sub4sub because they think YouTube gets easier when you reach a certain sub count.
YouTube does get easier as you grow, because of more people sharing your content... and also because greater watch times/engagement cause YouTube to see that your content may be worth watching, and recommend it to others/rank it highly. YouTube doesn't get easier because of you having a certain sub count, it gets easier when you have a certain number of active viewers (subscribed or not).
The problem is, doing sub4sub is actually the antithesis of this. By artificially inflating your sub count with people who are only subbing because they want you to sub back, you're getting a ton of dead/inactive subs who don't watch your videos. This just tells YouTube that you have a huge subscriber count but tiny viewer base.. which suggests that something is wrong with your videos that is causing no-one to watch them. As a result, if anything, sub4sub will actually make your videos rank and perform more poorly than if you didn't have those subscribers in the first place.
Also, personally if I see someone offering sub4sub, it doesn't matter whether it's on my video or elsewhere but I'll immediately report their comment as spam. I'm certain I'm not the only one, and if you're being spammy like that and regularly getting reported for it, that's going to cause you other issues. One notable thing that could do is cause your comments to automatically be flagged as spam.
I know I sometimes see certain users on my channel automatically going into spam when they comment - and all their comments are genuine and not spammy at all. A couple of times I've noticed that these are the same users who're going around and posting sub4sub requests on other channels. YouTube has seen them spamming, and acted accordingly to block it - so now they can't even have genuine conversations elsewhere.