No.  It was forwarded to me by another CPA who is part of the MD Comptroller forum or something like that.  It's only an hour or two old so the Comptroller's website isn't updated yet.  See below.

The Comptroller’s Office is in process of creating a new form, 511, for SOME pass-through entities = if your clients are impacted, they request that you PLEASE DO NOT FILE USING THE OLD FORM 510 BUT WAIT FOR THE NEW 511 which will apply to:

• if the client plans to make the election for Maryland PTE tax with respect to shares of ALL members (i.e. in order to take the position on the member’s federal return that the state tax is an entity tax and is therefore not subject to the $10,000 SALT deduction cap on the member’s individual return)

OR

• if the client had previously planned to make the election and therefore paid in tax for the entity, but now after further review the entity DOES NOT want to make the election (e.g. some have researched the treatment in the nonresidents’ resident state and determined that the credit treatment is harmful)

If the client is not impacted in either of the above categories, i.e. they are paying tax only with respect to nonresidents WITHOUT the new election (and the tax will therefore be treated as “on behalf of” the nonresidents (aka withholding) )= you can file using the existing 510 and don’t have to wait.

Still some complications to work out with respect to the new 511 and several of us are working on that with the Comptroller’s Office. Yes it has turned out to be way crazier than anyone had ever envisioned. Hang in there. We’re all working as fast as humanly possible.

Note that the whole premise of the Maryland change was to BENEFIT the client with a bigger federal deduction and therefore federal tax savings. You may want to remind them of that when explaining the delay and the possible desire for extending the returns.