How do bun, deno, and other typescript-built-in languages keep up with language features?
typescript(tsc) is the only one that does type checking.
bun, deno, esbuild, swc etc. can parse the syntax, but they chuck the TS (they probably don't even add it to the AST, but I haven't checked).
Keeping up with syntax is very doable. It doesn't change often, and updating the parser when it does isn't much work.
There are some past/ongoing projects[1][2] to create type checkers faster than tsc, but they aren't going to reach full parity and probably don't plan on keeping up with language features.