I want to release a new package as a successor to a package that has not been maintained for several years

[thep0y]

The author of the old package, although still active, has not maintained this package for 4 years. The availability of this package has become an issue over time.

I want to release a new package based on this package, fix some fatal bugs, and add some new features.

The new package is only released for ST4.

Is this allowed?

[idleberg]

Some thoughts on this practice: #8595

[FichteFoll]

In general, yes, we do allow package takeovers per the process that is discussed in #8595 (albeit without much input so far because it's a rather significant discussion and potential change to have). Our usual procedure can also be viewed in the review guidelines, but in general takeovers are allowed if the previous maintainer agrees or does not reply within ~2 weeks to a takeover request.

As for your case in particular, the package does indeed seem to be abandoned, seeing how it's been broken on ST4 with a trivial fix available for over a year now: adambullmer/sublime_docblockr_python#49.