Call me old fashioned, but I still just use note cards. Obviously portability is an issue, but I don't travel much (1 long trip a year pre-covid), so never ends up being a real problem.
Note cards are great, but I think a really big factor with a good spaced repetition system is to be able to essentially pick the best card to show you at any time. Based upon your history, which card you are most likely to have forgotten and need a refresher on, and would be most beneficial to show you next.
This would be practically impossible with a physical system. In addition to the fact that your memory of a new cards fades faster than cards you saw a long time ago; taking the time to approximate such a system would be a large burden that software can just handle for you.
That's what I used when I was learning Mandarin Chinese in college, pre-smartphones.
But I did my own version of spaced repetition -- when a card was really hard, I'd put it back in the deck just 5 cards back. If it was easy, I'd put it all the way in the back of my ~200 cards, or eventually remove it entirely. Or wherever it was in between, I'd slot it at an appropriate distance. It actually worked really well.
I did something similar for both Spanish and Latin. I'd grab 20-50 cards to study from the top of the stack, work through them and repeat if needed. Then shift them to the back. The very easy ones ("ser" or "ir", for example because they're seen so much) would get moved out of the deck entirely, and only ever seen again for a full-deck review. New terms or concepts (conjugating for different tenses, for example) would go on top. I ended up with 1k cards in Latin over two semesters. Not sure how many for Spanish.
