The way its worded here is that its an existing rule
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Just done a bit of digging and that seems to be the rule and for this season UEFA actually list dates when showing games at 3pm on a Saturday would be allowed but I'm pretty sure that wasn't taken advantage of.
Couple of interesting points and potential wiggle room, although the general consensus seems to be the whole blackout concept is on borrowed time, possibly going when the new PL deal is negotiated as there will be pressure from broadcasters to make all games available.
The relevant authorities have to provide UEFA with the time for the blackout and the time should be when the majority of the games in the top 2 divisions are played but a lot of PL games already don't kick off at 3pm Saturday and with the new EFL deal an increasing number of championship games won't. The rules don't say what happens if there isn't a single time when the majority of games kick off.
The other interesting part is that pretty much fuck all is likely to happen if the rule isn't enforced. The only people who can even lodge a complaint are "any member association which has incurred damage as a result of the violation".