This is a false dichotomy
It isn't fair to call spell names "flavor text", as if they are meaningless or not valuable — but at the same time it's not like the names [Leomund's] secret chest or flamestrike are in some way rules. I feel like this is drawing a distinction that doesn't really exist.
Spell text is — in theory — rules text, but it often includes elements like specific gestures (such as touching thumbs to cast burning hands) or specific visual effects (such as a sacred flame "descending" onto the target or faerie fire being specifically green, blue, or violet) which could be easily replaced without changing the functional, mechanical effect of the spell. Tasha's Cauldron of Everything even has a whole section about customizing the visualization of your spells.
The same token, spell names are technically not "rules text", and can be easily replaced to fit a different theme, style, or concept; but they do serve a purpose in helping us understand what the spell text is getting at. In many cases, the intent of a spell is not clear if read in isolation, without the spell's name to help guide our understanding of the mechanical effects.
As an example: as long as everyone at the table understands that your chronurgist's temporal acceleration is actually the haste spell, the name doesn't matter — but at the same time, if we didn't already know what the spell was meant to be doing and it was named something like "Geryon's Enhancement", it wouldn't necessarily be obvious that "double your speed, increase your AC and dex save, and take an extra action" is all trying to represent that you're just moving faster than normal. The name of the spell helps us understand the spell effect, but is not a critical mechanical element of the spell.
I would argue that spells as whole meld rules and flavor together so that you can't pick out particular sections and say "this is but that isn't". A spell is a bundle of rules text, but has flavor elements sprinkled through it, and it's up to you as a DM or player to identify which parts can be safely changed or reskinned and which ones could affect the game if altered.