I briefly looked into it (from the same NixOS issues), there's a GitHub issue somewhere about it [1]

Effectively as I recall it a large amount of Sublime Text plugins internally rely on an old version of Python (3.3) which in turn relies on OpenSSL 1.1.1, there is concern that forcing the python version to something newer will largely break their plugin ecosystem as a lot of the plugins are expecting Python 3.3 and may not be compatible with 3.8 or later versions with supported OpenSSL.

It's probably not a major security risk unless the plugins are making network connections, but it is an unfortunate situation.

Edit: [1] https://github.com/sublimehq/sublime_text/issues/5984