The problem with “modern” in that title is that it isn't clear what it means now, and isn't future-proof — twenty years from now that question will exist here or in a database of “RPG Knowledge” somewhere, and it won't make any sense. We write for posterity here, so questions should be evergreen.
It's also overcorrecting for the tag-in-title problem. Normally, we avoid putting “tags” in titles, but that refers to specifically artificially adding tags to titles and writing things like “How do I frob the fibitzer? [The Dark Eye]” or “In Paranoia, how can I earn a higher clearance?”:
Should I use tags in titles?
You should not force a tag into your title. Because the tags appear below the question and tags are indexed by search engines along with the content of your question, you can trust that other people will be able to find your question based on tags they follow or search for. Additionally, tags appear on the question page, so other people will take them into account when answering your question.
Avoid inserting tags into titles in any of the following formats:
- [tag]: [question title]
- [question title] -- [tag] [tag] [tag]
- [question title] in [tag]
- [tag] [tag] [question title] [tag] [tag] [tag]
- [tag] [tag] [tag] [tag] [tag] [tag] [tag] [tag]
The only time you should use tags in your title is when they are organic to the conversational tone of the title.
That last sentence is important though! Things that match our tags do belong in the title when it's the most natural way to write the title. So in the case of your question, “Is there a modern equivalent of the e6 variant style?” is overcorrecting by avoiding even mentioning the game system, rather than merely avoiding slapping tags into the title. “Is there a D&D 5e equivalent of the e6 variant style?” is organic and clear, and doesn't unnecessarily force a tag into the title — it naturally belongs there.
To demonstrate by argument ad absurdum, removing “D&D 5e” from the title by force is like removing “e6” (also a tag) from the title: forced and unnatural, and possibly resulting in something like “Is there a modern way to maintain the sweet spot of of power?”, which would be contorted, unclear in the question list, and nearly impossible to search for later.