Is LGPL-2.1 compatible with LGPL-2.1-or-later?
Yes.
Same license line: the 'or later' work can be used under LGPL-2.1-only, so the two combine.
Result license is the more restrictive of the inputs. You must satisfy every input license's notice and source-offer terms. Verify pairwise compatibility first.
Resulting work's license: LGPL-2.1-only
General guidance, not legal advice.
Obligations when redistributing as open source
- LGPL-2.1: retain the copyright notice and license text.
- LGPL-2.1: state significant changes made to the code.
- LGPL-2.1: publish the source of any modifications to the library itself.
- LGPL-2.1: modifications to the library must remain under LGPL-2.1.
- LGPL-2.1-or-later: retain the copyright notice and license text.
- LGPL-2.1-or-later: state significant changes made to the code.
- LGPL-2.1-or-later: publish the source of any modifications to the library itself.
- LGPL-2.1-or-later: modifications to the library must remain under LGPL-2.1-or-later.
Other distribution models
The verdict is the same under every distribution model; the obligations differ.
Check this pair in the tool to switch models.