Yes it is intentional i think.
Domesticate Elephants was axed and captured Elephants can now be upgraded at as early as Horseback Riding and have been nerfed from 8 to 7 strength. (Stables might be reqiured don't remember just now. As does ivory i think.)
They are quite good now if you aim for Horseback Riding / Trade early and can catch some wild ones.
Quite a good development i have to say. (has made the Elephants from the least wanted prize / hated nusiance to one of the most interesting ones and reduced an unneeded Tech from the Tree.)
They keep their HN Promotion if the still have it. Nice for some wild raiding early on...
Especially for Balspheraphs i reckon. Build Loki, capture some Elephants (shouldn't be hard if you really aim for it) and Mutate them. then release / axe the unwanted / enraged / crazed ones or play some experience ping-pong with them (can't attack anyways so no danger there) since at least if they are crazed you can use them for EP-sparing over and over again (by disbanding them while they have the Enraged promo) with a subdue animal Unit. (I don't know if it works as reliably with just enraged, but crazed works for sure)
and upgrade the best-mutated ones to real killing machines (canibalizing (very important for that trick i reckon), heavy / empowered / Strong / Herooff/Herodeff Elephants + Riders with Blitz perhaps also Combat 1 or Mobility 1 and Light for extra Movement are really fearsome at that stage in the Game. And as soon as they reach commando (which shouldnt be to hard, espacially if you "pre-feed" the wild elephants pre-capturing for easy subdual) your oponents should be toast + your coffers brimming with money. Until Champions nothing could be able to match their power even remotely.
At Epic Speed this should be enough to clear a map before the oponents can field something which can resist them in a meaningful way.).
Mutate as a "Mage-Spell" is wrong anyways imo and should be replaced with the chaos marauder again / given Back to OO.
But i have to admit it really fits the Balspheraphs Flavor very well.
"My Lord...
theres a 4 tusked, 8 legged monstrosity marauding our borders...
Our Garrisons are in wild retreat...
What shall we do?"
"Must be coming from those mad clowns next door..."
So why deny them anyone else? Worse than that they won't really become. Vampiric Elemphants take al lot more time to pull of and much more effort. Then they are not as powerful anymore. (and don't get the Promos one can gain from mutate.)
Strength 5/6/7 Cavalry (which is comparable not like the 15 or so strength of the old elephant riders) should be nearly available around the time you can reach Feudalism.
Might not be perfect from a lore perspective for all folks but seems balanced (if Balspheraphs with mutated E-Riders are in the first place

which may very well be not).
Though it could be that civs that can't build stables (does any civ?) cant upgrade them.