This concept was very popular back in the days when computers used to boot from HDD, but now it doesn't make much sense. I wouldn't notice If my laptop boots for 5 sec instead of 10.
At the time of their introduction Optane drives were noticeably faster to boot your machine than even the fastest available Flash SSD. So in a workstation with multiple hard drives installed anyway, buying one to boot off of made decent sense.
If they had been cheaper, I think they'd have been really, really popular.
By my reckoning, there was zero overlap between the period of time where a reasonable computer configurer would pick a hard drive to boot from and the period of time where Optane was available.
And even for the general concept of a cache drive, I don't think I've ever seen it do well in the mainstream. Just a few niche roles, and some hybrid drives that sucked because they had small flash chips and only used them as a read cache, not a write cache.
If they had been cheaper, I think they'd have been really, really popular.