Actions and Codespaces have been huge and transformative, so it's disappointing that core functionality like code reviews hasn't seen the same level of improvement.
Basic features that Phabricator had nearly a decade ago are still missing: - highlighting copied/moved code blocks - gutter indicators for code coverage - stacked diffs - not collapsing large changes by default (what a ridiculous default! "A lot of code has changed here, meaning it's likely bugs may be here - I'll hide that for you to make your review easier!")
And I don't recall how well Phabricator supported it, but handling rebases sanely by carrying over comments and showing diffs from prior PR versions across rebases would be amazing. The number of times I have to re-review an entire PR in GitHub because it can't show what changed since the last review if the author rebases their branch...