I mean, now you've brought it up, I am concerned about it - but the level of concern is somewhere between "spontaneous combustion of myself leading to exploitation of my domain DNS because my bugger-i-ded.txt instructions are rubbish" and "cosmic rays hitting all the exact right bits at the exact right time to bugger my DNS deployment when I next do one which won't be for a while because even one a year is a fast pace for me to change something."
(Plus I'm perfectly capable of taking my sites and domains offline by incompetent flubbery as it is; I don't need -more- ways to fuck things up.)
There are also good reasons many serious admins don't trust signing authorities. If you know... you know why... =3
Yes, exactly.
On a busy site, the incurred additional load cost can bite hard.
A lot of people will leave it off for the same reasons as DoH or DoT. =3