Probably MOST of them are, but certainly not all.
It's worthwhile to read the entire contents of this link.
https://www.atf.gov/rules-and-regul...armswithattachedstabilizingbracespdf/download
"On June 10, 2021, the Department published an NPRM in the Federal Register titled, “Factoring Criteria for Firearms With Attached ‘Stabilizing Braces’,” 86 FR 30826. The NPRM proposed amending ATF’s definitions of “rifle” in 27 CFR parts 478 and 479 to expressly state that the term may include firearms equipped with a “stabilizing brace,” even though such firearms were already implicitly included in the definition by virtue of the fact that they were designed, made, and intended to be fired from the shoulder."
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"The revised definition in this final rule clarifies, consistent with the best interpretation of the statutory provision, that firearms with an attached “stabilizing brace” can possess objective design features that make them “rifles,” as that term is defined under the NFA and GCA. If a firearm with an attached “stabilizing brace” meets the definition of a “rifle” based on the factors indicated in this final rule, then that firearm could also be a short-barreled rifle depending on the length of the attached barrel, thus subjecting it to additional requirements under the NFA and GCA. However, a firearm with an attached “brace” device is not a “rifle” as defined in the relevant statutes if the weapon is not designed, made, and intended to be fired from the shoulder. The rule, as proposed and finalized, does not ban “stabilizing braces” or prohibit firearms with an attached “stabilizing brace,” regardless of the firearm’s classification."
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"While firearms equipped with “stabilizing braces” or other rearward attachments may be submitted to ATF for a new classification determination, a majority of the existing firearms equipped with a “stabilizing brace” are likely to be classified as “rifles” because they are configured for shoulder fire based on the factors described in this rule."
If you want a stabilizing brace on your pistol then you can have one. It needs to really be a stabilizing brace, and not a shoulder stock that is masquerading as a stabilizing brace, or even a true stabilizing brace that is designed so that it contains features that are common to shoulder stocks but not required in a stabilizing brace. Reading the entire document in the link helps explain this.
What is over with as of this rule's issuance is the use of "stabilizing braces" (note the quotes) to circumvent NFA restrictions. If you want to have something that is designed to function as a shoulder stock on your pistol then you need to go through the NFA hoops.