chore[tags]: Change some methods from private to protected, to be able to extend the affected classes

[iPurpl3x]

Changes proposed in this pull request: see title

Necessity

• [x] Has the problem that is being solved here been clearly explained?
• [x] If applicable, have various options for solving this problem been considered?
• [x] For core PRs, does this need to be in core, or could it be in an extension?
• [x] Are we willing to maintain this for years / potentially forever?

Confirmed

• [x] Backend changes: tests are green (run composer test).
• [ ] Core developer confirmed locally this works as intended.
• [x] Tests have been added, or are not appropriate here.

[luceos]

@iPurpl3x out of curiosity, did you check the check boxes intended for maintainers? 🙈

[iPurpl3x]

@luceos I guess I did, but it wasn't clear to me whether I or a maintainer should check those checkboxes…