As mentioned previously, a pull request was suggested by the person looking after the hardware, although as you say there are old PRs from a few years ago
It would appear that someone (perhaps the OP here) has created 3 issues there, but based on what we’ve seen so far I suspect that it would take some time (a bit of education or self-learning about how these things work) for the OP here to create a PR.
If someone was really keen to add highway labels to this map style I’d suggest the best way forward would be for someone to fork the style and deploy it somewhere, but that’s likely beyond the OP here, at least for now.
The good news is that there are guides for setting up servers for similar map styles, and once that’s been done it’s possible to use a different style, and to change that different style slightly.
Sightly tangentially, it’d be good to see a guide for “creating a map style that uses the OSMF vector tiles”, because that’s a lower barrier to entry - although even that requires familiarity with at least JSON and Javascript. The relevant documentation is “somewhat challenging”, especially with regards to icons, sprites and fonts, so it absolutely makes sense to experiment with a “simple” style first.