For the record this isn't an official Tempus utility (tempus-demos.xyz is my site and while I am on the Tempus staff it's not "officially" connected).

This tool is a WASM compiled version of the demos.tf re-encoder which has been available for some time now, if you want to bulk process a lot of demos or anything it would probably be better to compile it yourself - https://codeberg.org/demostf/parser/src/branch/master/src/bin/reencode.rs

This has been tested on quite a large number of demos from Tempus dating from before 2023 and when bugs were found we contributed fixes so the coverage should be pretty decent. We have had demos working as far back as 2014 but haven't tested earlier as Tempus doesn't have records from before then, so it may or may not work for even older demos.

One notable exception is voice, the voice codecs used in TF2 have changed a few times over the years and while the data in the demo is perfectly fine and extractable, current day TF2 often doesn't know how to play it back (Valve made some bad choices when updating this part of TF2), so it will sound garbled.