@esm

(All of the below apply to GIF. ✅ and ❎ indicate whether they apply to MP3 as well.)

Been around since the 90s - ✅
Still in use in a distressing number of parts of the Internet despite widespread availability of much better codecs - ✅
Noticeably inferior to those codecs - ❎
Has a reputation for low quality - ✅
Has a reputation for low quality because the only thing people ever use it for nowadays is something it was never designed to do - ❎
Open source - ✅
Supported by everything under the sun - ✅
Originally designed to be either of those things - ❎❎❎
Lossless - ❎
Files are bloody massive for what they are - ❎❎❎
Really, REALLY lacking in features - not to my knowledge
Suffers from a thankfully relatively mild case of Extensible Format Syndrome™ - ✅
Synonymous in the common lexicon with any computer file that does what it does, regardless of the actual codec of that file - ✅

Conclusion: MOSTLY TRUE!