Did not know until reading this that there is an upper-limit on the number of HTLC's per on-chain tx (483). Is there any mechanism in LN implementations that close channels once this number is hit? Or is that even necessary?
Warning: this is mostly from memory and deduction than me referencing an implementation.
Closing channels shouldn't affect this number - these just apply to pending HTLCs afaik and a peer won't forward any more HTLCs once this number is hit (at least this is how I image an impl handles the condition). I might be mistaken but I think this limit relates to how many outputs you can fit into an on-chain tx. Once HTLCs settle, new commitment txs are formed between channel peers, ie theses HTLCs are integrated into their channel balance rather than being a separate output, so they can begin accepting more again.
afaik PTLCs primarily help with payment de-correlation along a route - all hops see a different preimage rather than share one like the current version. There might be further ramifications but it's mostly a privacy and security enhancement.
Warning: this is mostly from memory and deduction than me referencing an implementation.