At least since 2021 that's what the instructions have required, if the client wants to grant authority to sign returns.  I don't check that box because I represent clients, I don't sign their returns (other than as preparer).  As the 2848 instructions point out, Treasury Regulations require the taxpayer to give a reason.  Do your clients really want you signing their returns?  Why do you want that authority?

Authority to sign your return. Treasury Regulations section
1.6012-1(a)(5) permits another person to sign an income tax
return for you only in these circumstances:
(a) Disease or injury,
(b) Continuous absence from the United States (including
absence from Puerto Rico) for a period of at least 60 days prior
to the date required by law for filing the return, or
(c) Specific permission is requested of and granted by the IRS
for other good cause.
The IRS may grant authority to sign your income tax return to (1)
your representative, or (2) an agent (a person other than your
representative).