There are NHL bylaws which predate the CBA.
Many of those bylaws have been incorporated into the CBA over time, with changes/updates as part of negotiations.
Some bylaws remain in affect in addition to the CBA. The relevant bylaw which prevents players traded post deadline from further league participation in that year, is still in effect as originally ratified.
(See Allan Walsh twitter thread where this was more or less confirmed. Though no official public source is readily available.)
That bylaw prevents any player moving from one club's reserve list to another, after the deadline, from participating in any regular OR playoff games for the remainder of the season.
(Note, UFA's signed past the deadline are eligible for regular season, but not playoffs, as they were not on another club's reserve list after the deadline.)
Players CAN be traded, but that means the end of their season. This is why we normally only see non-playoff teams making trades once the playoffs begin.
Teams normally aren't going to trade players and end their seasons early, or trade for players that they can't play.
Quoting: Jah1722
Players traded post deadline are ineligible to play for rest of the regular season and playoffs. If VGK trades Dadonov and forces him not to play another game this season the NHLPA will file all kinds of complaints against VGK.
Dadonov and the NHLPA will have no grounds for a complaint if the trade is nullified and Dadonov is moved elsewhere or waived. It's a legitimate option for Vegas and not relevant to the original trade.
I'd like to see Dadonov granted a contract termination at full value (Vegas pays his remaining salary) and classified as a free agent, making him eligible to sign with any team to finish the regular season, though he'd still be ineligible for playoffs.
Whoever is determined responsible will then be penalized with pick forfeiture and fines, and on the hook for the salary paid out to Dadonov for his termination.