HK Bar spells out worries regarding the Immigration Amendment Bill:
“The regulations to be made under the proposed new section 6A(1)(b) would, however, authorise the Director to prohibit any person, including Hong Kong residents, from leaving Hong Kong.”
https://www.hkba.org/sites/default/files/Immigration%20%28Amendment%29%20Bill%202020%20-%20Clause%203%20-%20Further%20Submission%2011.2.2021.pdf

“The HKBA is deeply concerned about the proposal to confer an apparently unfettered power on the Director to prevent HK residents and others from leaving HK. It is particularly troubling that the grounds on which such an intrusive power may be exercised are not stated”

“If a new power to prevent HK residents and others from leaving…is to be conferred, the grounds on which it may be exercised must be stated clearly in the primary legislation, and it should be for the Courts, not the Director, to decide when it is necessary and proportionate”

A previous submission from the HKBA was also very critical of the Bill's alleged main goal of streamlining asylum/torture claims
https://www.hkba.org/sites/default/files/Immigration%20%28Amendment%29%20Bill%202020_0.pdf

The draft bill can be read here: https://www.elegislation.gov.hk/hk/2020/12/04/supp3/1!en

And the issue was previously flagged by @maryhui :
https://qz.com/1952305/a-new-immigration-law-could-impose-exit-bans-on-hong-kongers/
/end

Good thread on the government's response to concerns: https://twitter.com/aaronMCN/status/1360559144172982282?s=20