I had major dental work done over 2 yrs, bill was rec’d and insur filed in the 2nd yr. I prpd 1/2 in 23, 1/2 in 24. Can the unreimbursed prepay be deducted in 23 or 24?

Deduct it in the year you actually paid.  So, half in 2023 and half in 2024. 

if you filed an insurance claim and that will reimburse you for some of what you paid then then you are subject to the rule for recoveries (IRS PUB )

A recovery is a return of an amount you deducted or took a credit for in an earlier year. The most common recoveries are refunds, reimbursements, and rebates of itemized deductions. Tax benefit rule. You must include a recovery in your income in the year you receive it up to the amount by which the deduction or credit you took for the recovered amount reduced your tax in the earlier year. 
lier year