I'm such a fan of pinned messages for this reason.
I'd love if I could re-forward a text message to myself in a day or two. Or if there were some other way to remind myself of messages within the text message app.
I totally agree. Pinned messages are very effective, moreso than taking a screenshot or a mental note, both of which will be forgotten about relatively quickly. A calendar entry might be more effective, but the context switch to a different app makes this unsuitable when low-friction is required.