I didn't do much research, and the way I saw references to Specialist in articles made me think they were used more recently through 7. I never saw references to 8-9.
They were authorized in the 'Nam era... but only NG and Reserves got to use them.
I've met one Sp8 ret... he was a cook. Retired, then got the civil service job running a mess hall at Dix...
Note that the initial specialist grades were limited to E7, during the Korean War period... but they were a replacement for the Technical ___ ranks of the WW II era, where T in the center void means "Not an NCO, just a technician"... which was confusing to many. ANd, at the time, a corporal technically had rank over a specialist of any grade.
So, the splattered eagles were devised, as they no longer looked like standard Corporal and Sergeant grades.
Then, around the same time, supergrades were added...
But towards the end of the 'Nam war, the Army decided there were too many "speedies"... and started trimming the Sp7 and Sp6; any on active got lateraled to an NCO of same grade. (Most of them were functionally NCOs without BNCOC and ANCOC). So... the 5's were retained for some reason until the early 80's and I've never seen it explicated why.
Byt the late 80's, most of the Sp5's had been encouraged out to to BNCOC
For some careers, being a veteran should be a positive bonus to enlist, such as bureaucracy and law enforcement, besides the normal security and and mercenary billets.
I'd honestly rather not see a retired combat arms soldier get preference for police jobs; we just can't afford the "it's a war" mindset in police.
Plus, a lot of vets have PTSD, which is not a good thing for a police officer.