Doesn’t that just lead to an echo chamber on the site they left also? When Trump supporters were banned from the political subreddits, they formed /r/The_Donald. And it’s an anti-Trump-criticism, anti-anything-left-leaning echo chamber. But that left /r/politics to become a left leaning echo chamber itself. Now, one can’t have a civil discussion about anything because /r/politics will ban anything saying Trump did anything remotely good, and /r/The_Donald will ban anything remotely critical of Trump.
This is a false equivalence - r/politics is not the left leaving equivalent of r/The_Donald. Trump supporters weren't broadly banned from political subreddits, just the ones that broke the rules did, like everyone else. r/politics is a left leaning echo chamber, yes, but you won't get banned for being pro Trump (just downvoted). On the other hand, you will get banned from r/The_Donald or r/conservative for being anti-Trump.