tl;dr:
curl https://inoue.szy.lol/api/replay/acc91d171d3b1ae6b928c3bc8 -o mycutereplay.ttr # the hex string is the TETR.IO replay id
that simple!

successful requests return 200 and the replay with the application/octet-stream mimetype. unsuccessful ones return a plaintext error explanation as text/html and a proper HTTP status code - 404 for not found replays, 429 for rate limits, 50x for server errors.
both long and short replay IDs are supported. do not include the R: prefix you see in tetrio replay links.
most requests are forwarded to the tetrio api, so please don't spam. if you get a 429, then back off.
do not depend on the replays staying up here after they are deleted on tetr.io's side. they are cached, but they will be deleted after some time.
i reserve the right to laugh at you in the tetrio discord Programmers Thread if your client's conduct is egregious, your IP will be redacted though

not a developer? if you're only looking for a program to auto-save your tetrio replays, check out Inoue! the name is not a coincidence, this website works as a backend for it.

contact: inouebot πŸ„°πŸ…ƒ szy πŸ„³πŸ„ΎπŸ…ƒ lol, alternatively szy#7522 .szy. on the gamer website
this service is not affiliated with TETR.IO or osk. it is provided in hopes it will be useful, but with no warranties, express or implied. no SLA, but feel free to tell me if something doesnt work.