My 2c:
As is so often the case, the thoughtful answer is "it depends." And it depends not merely on the system, including room and power considerations, but also on the listener’s goals, attitude, and ability to hear (and tolerate) sonic compromises.
In the past, EQ meant profound phase shifts. In a carefully time-aligned system -- whether it consists of DCM Time Windows or Wilson Chronosonics -- an outboard equalizer may compromise the system’s meticulously crafted design. But if you’re listening to midfi cones in an untreated room, the adverse FX of even a medicore equalizer may be inaudible.
Newer EQs, of course, incorporate more sophisticated designs that (I’m guessing) mitigate the adverse effects of the cheap Realistic boxes that were popular in the 70s. I can’t speak authoritatively on the subject, but I’d guess that software equalizers may be even less likely to pollute the audio stream (at least in some ways).
In my own experience, I’ve done some pretty drastic upgrades over the last 40 years, some of which would make my hair rise today -- such as a homebrew EQ that consisted of installing a 100w 6ohm resister across my Quad ESLs to reduce the teeth-grinding high-end of an early Quad solid-state amp -- and often found that any audible artifacts were outweighed by the improved frequency response.
That is, until I got smart enough to buy kilobuck cables, invest in effective room treatments, and install $8000 of Audioquest Niagara power conditioning and AC cords. Once I did all that, even the smallest tweaks, like fine-tuning my cartridge’s true rake angle after having the stylus photographed by WallyTools, made a clearly audible diffference. I suspect that in my current system, almost any outboard EQ would generate undesired FX, especially re: parameters dependent upon accurate reproduction of tiny details, like soundstage, imaging, and room ambience.
Having said that, there are situations in which I would still consider using EQ: to correct the frequency response of otherwise-excellent older recordings, such as those transcribed from 78s, or even a later-period recording that is steely or that has boomy midbass. And even then, I don’t think I’d go for any EQ unless it was designed by a person or company that has clear audiophile intent and skill set -- such as an EQ module or tone controls in a four- or five-figure preamp or integrated sold by a trusted first-tier company. And I’d have to be convinced that, when disabled, the EQ function is truly taken completely out of the signal path.
But I respect the fact that other people’s specific situations can vary. Again, the correct answer is "It depends."
One thing I’ve learned in life is that simple questions tend to require longer responses. A generalized query that doesn’t state boundary conditions often requires a detailed answer that enumerates answers for multiople use cases. A detailed question that precisely describes the scenario to be addressed can often be answered more simply. A Twitter-length answer to the OP’s question might be telling you more about what works for the responder than it does for the OP.