Can an SM wear a regimental affiliation unit crest even if unit has been deactivated?

I do. I wear the 313th MI BN, 82D ABN crest, although it deactivated in 2006. The reg doesn't specify anything about unit deactivation being a factor. In my opinion, if the SM served in the unit, that crest is associated with them for the rest of their career.

According the para 19-23a, DA PAM 670-1,

a. Authorization. AR 670–1 authorizes insignia used to represent regimental affiliation to consist of either the RDI or DUI. A Soldier’s Regimental affiliation using an RDI is based on a Soldier’s branch/corps/special branch as determined by PMOS or specialty. A Soldier’s Regimental affiliation using a DUI is based on a unit in which the Soldier is serving or previously successfully served. Soldiers may wear the RDI for their affiliated regiment or may wear the DUI for a unit in which they are serving or have previously successfully served based off their assignment history as indicated in their personnel record.

SFC (Retired) Efrain Hernandez As long as you meet the requirement in AR670-1. It doesn't say active or otherwise. Units deactivate all the time and the CMH keeps an OML of each so when reactivations come around, their lineage and heraldry are restored.

AR670-1: "19–23. Insignia representing regimental affiliation
a. Authorization. Insignia used to represent regimental affiliation consists of either the RDI or DUI of a design ap- proved by TIOH. A Soldier’s regimental affiliation using an RDI is based on a Soldier’s branch/corps/special branch, as determined by PMOS or specialty. Soldiers may wear the RDI for their affiliated regiment or may wear the DUI for a unit in which they are serving or have previously served successfully, based on their assignment history as indicated in their official personnel record.
b. How worn. See DA Pam 670–1."

The TMDE DUI is actually for one of the Battalions. I could wear that one as I successfully served as the Commander of 95th Maint Co, which bore that DUI. Neither that BN nor 95th exist anymore

If the affiliation was approved prior to the unit's deactivation, yes.
As long as there is still one active numbered BN of a regiment to approve affiliations, they can still be approved.
Once that last numbered BN is deactivated, there is no longer an authority to approve affiliation to that regiment.