Samsung did restrict side-loading recently,

- https://news.ycombinator.com/item?id=47202808

I'm sure that Google will do something like that as soon as it faced the US's carrot and stick they signed-up for.

That's not really sideloading, though. The stock recovery doesn't let you install apps or anything like that, it's meant for loading official versions of Samsung operating systems onto devices that got corrupted somehow.

You can probably try to use the stock recovery to flash a custom ROM, but I doubt it'll work. Custom ROMs rely on tools like TWRP or LineageOS Recovery for a reason.