It happened in an old Marvel Star Wars comic. Three ISD-I's came out of hyperspace directly onto the
Executor and rammed it. The Executor's shields held and the ship itself suffered no damage.
Also it wasn't even a "tactic." The Imperial Admiral tried to execute a microjump to catch the Rebel Fleet between two forces but instead due to stellar interference the Imperial Star Destroyers came out of hyperspace directly into the
Executor.
Thus Admiral Purple Hair Holdo is the single greatest mind in the fifty or hundred thousand years or whatever of Star Wars space combat confirmed for discovering and executing the Hyperspace Ramming maneuever. It's called the Holdo Maneuver for a reason after all.
If she wasn't the first person to come up with this genius tactic, it wouldn't of been named after her.
Cope and Seethe Imperial MAGAts! Women can finally do things in Star Wars too.
Yup. This was in the old (I mean OT-in-the-theatres era) Marvel comics, and it happened
once. The 'ramming' ships exploded, the 'hit' ship... was unharmed.
That's pretty much sensible, and exactly what you'd expect. The ships have to "drop back" into realspace, and can't. So they don't even really hit you at all. Their just explode upon re-entering realspace... apparently without enough force to penetrate a decently strong energy shield.
I could believe that if you did it to an unshielded ship, you could do some real damage. But even then... an ImpStar is (taking the Executor's
maximum size estimate!) a twelfth of the SSD's size. And this is three of them. Meanwhile, the Raddus is less than one seventeenth of the Supremacy's size. If these ImpStars can't even overload the shield, then using that logic, the explosion can't be all
that powerful, and the TLJ explosion ought to be proportionally weaker.
(Meanwhile, the TLJ explosion destroyed
twenty other SDs, canonically, including ones that are
outside the 'debris cone'. They got cut in half with a weird light-effect, due to some magical hyperspace effect. Yes, hyperspace ramming canically destroys other ships in the target region as well.)
We may thus assume that in Disney canon, hyperspace-exit booms are
way more powerful than in the old continuity. So much more that this tactic is a viable one-shot fleet-destroying move, which would furthermore allow a decently big ship to (for instance) at least render the Death Star's superlaser very damaged and inoperable.
It's also not "one in a million", since we see it happening
again during one of the finale shots in TRoS (over Endor, I think). That's the
same film that claims that it was "one in a million"!
So it's absolutely, unquestionably lore-breaking. The introduction of this notion renders all other space battles in the entire setting useless. Even if you have to sacrifice a lot of ships because it
often goes wrong... that's worth it. Hyperspace ramming would be
the number one successful tactic. All space battles would be centred around the aim of getting your forces in such a position that your big 'ram-ships' can do
this specific thing.