Here is my view on what these tags stand for:
- spaced-repetition is for the technique, regardless what "technology" you use for it, flash cards, a (digital) spaced-repetition system or something else.
- flash-cards refers to either paper or digital flashcards (or flash cards). The tag is close to the previous one but not identical, because there are spaced-repetition techniques that don't rely on flash chards, such as the "gold list method".
- memorization is for any memorisation technique and is therefore not limited to spaced repetition or flash cards.
I can think of very few topics where spaced-repetition would be appropriate but flash-cards wouldn't, e.g. the "gold list method" listed above. I can't think any questions where flash-cards would be appropriate but spaced-repetition wouldn't. If forced to reduce the number of tags, I would turn flash-cards into a synonym of the broader concept spaced-repetition.
Having anki seems a bit excessive, since each of the above tags would apply to questions tagged "anki" and it is easy to search for all content about "anki" that is tagged with