If it is destroyed and sent to the graveyard, but I Ritual Summoned it before, can I special summon it from the graveyard?
I mean, it says it "must be Ritual Summoned" but unlike Blue-Eyes Chaos MAX Dragon it doesn't say it can't be special summoned by other ways.
can I special summon it from the graveyard?
Short answer, no... The card says it Must be Ritual Summoned, so you can't Special Summon it unless it's a Ritual Summon. Longer answer...
I mean, it says it must be Ritual Summoned but unlike the Chaos Max it doesn't say it can't be special summoned by other ways.
I understand your confusion, and I was also confused at first. Seems that Konami recently changed the phrasing of such restriction on their cards, and updated it from the previous way of writing it they had. It is not uncommon for Konami to update or change their phrasing (but not the effect) to reduce ambiguity and shorten the text.
Let's see why analyzing MAX Dragon... Currently, there are two (three, but two of the same set) printings of Blue-Eyes Chaos MAX Dragon.
The first one is from a movie pack, code MVP1-EN004 released on 2016 which reads:
You can Ritual Summon this card with "Chaos Form". Must be Ritual Summoned, and cannot be Special Summoned by other ways. [...]
The second one is from the Legendary Duelists, code LED3-EN000 released two months ago (2018), which had it's phrasing updated (just the phrasing, though, not the actual effect). It reads:
You can Ritual Summon this card with "Chaos Form". Must be Ritual Summoned. [...]
This tells us that Konami now rephrased such part of this card's effect (and of others) so it is shorter. Now, cards that read "Must be X Summoned" are restricted to such kind of summoning only.
