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This has been addressed in the main YGO subreddit. Basically, there was an error during a maintenance and one of the Yugipedia admins accidentally erased all of the text files from the past few years. Images seem to have survived, however.
The point is, they are racing to rebuild the hundreds of the wiki entries that got erased, but Yugipedia will be down likely for a long time.
Why are you even using those wikis konami official card database works wonders, gives you the same info, it's 100% mobile friendly and has none of those intrusive video ads
Those wikis often offer tips and strategies to use for most of the cards and update spoilers of soon-to-be-released sets. Also, they have info on non-TCG aspects of the game, particularly anime-oriented. You know, trivia, character profiles, episode summaries...all that stuff.
^
Don't forget about the ruling sections. You can easily look up specific rulings of a card that might have an ambiguous wording or seem a bit confusing. Also, some of the archetype pages are helpful too because they recommend some other cards that might work well in that archetype.
It'd be very tacky to go to the wiki and find some entries are available and some are not. That's why they won't restore access till everything is back to how it was before the mess up.