I don't think it's just about champions having good mythic choices- it's also champions only having a good mythic for a prescribed gameplay style.
In terms of build diversity champions like Sona have a great mythic choice in moonstone but she struggles to be as effective building anything other than moonstone. Which is in sharp contrast to the prevalence of AP scaling Sona builds last season.
This is in part because of Sona nerfs but also because the lack of synergetic AP mythics she has access to: mandate, ludens, nightharvester, everfrost etc all have flaws like mandate only proccing off empowered E or R, and the others being way too expensive and not quite right for Sona. Also removing mana options outside of the mythics and seraphs (which is considerably worse than before- seriously can we change the build path to lost chapter + tear + amplifying tome) makes life harder for mana hungry AP Sona.
Sona is a big and obvious example but on a smaller scale playing supports with mythics just feels bad.
I play enchanters so it sounds odd to complain about mythics feeling restrictive when basically all enchanters built Athenes last season, but what Athene's did was enable (way too easily) enchanters to build other support items to strengthen heals/shields or to completely ignore the support item category for the rest of the game and just build lichbane, rabadons etc. Regardless of champion every enchanter synergised with Athene's and it had a unique gameplay (the stacks really should have decayed to make it balanced).
Imperial mandate and moonstone both end up being items that are good if your champion can abuse them rather than being options to choose from outside of playing champs that are just strong (like Lulu).
And I think the changes to support items adds to this restricted feeling:
The old zekes was an item picked up by both tank supports and enchanters or catchers (you ulted near your adc and they got a steroid) now it is a tank support item only, twin shadows was removed to stop abuse with glacial augment but now it can't be used to safely ward or check objectives, moving locket to a mythic means you can't buy a useful active to save your team from burst without sacrificing an item that synergises better with your champion if you don't want the aura passive. For a role that is focused on building items to buff your team and buying stuff to counter the enemy you can really feel the decrease in options.
And don't get me started on making support items build out of 3 slots not 2...