When did this change? I can tell you with almost certainty, that 2009 and prior, there was no such rule for undrafted, college UFAs.
Also, Danny Dekeyser absolutely played a couple playoff games for Detroit, as well as AHL playoff games, right after signing as an undrafted college UFA. That had to be 2013 or 2014.
As uncleben pointed out, it's purely a timing thing with the trade deadline. Typically college seasons wrap up in March for most teams unless they make the Frozen Four. But sometimes you get a year with a delayed start due to lockout or the pandemic seasons when the trade deadline got pushed back.
March 9, 2006
February 27, 2007
February 26, 2008
March 4, 2009
March 3, 2010
February 28, 2011
February 27, 2012
April 3, 2013 -- lockout
March 5, 2014
March 2, 2015
February 29, 2016
March 6, 2017
February 26, 2018
February 25, 2019
February 24, 2020
April 12, 2021 -- pandemic
March 21, 2022 -- pandemic
March 3, 2023
If Ben Meyers had signed before the trade deadline last year, he would have been playoff eligible for Colorado. But Meyers was competing in the Frozen Four in early April. A few NCAA guys signed before the 2021 trade deadline but most weren't the types to immediately jump into an NHL playoff roster (Josh Dunne, Alex Steeves, Jordan Kawaguchi, Odeen Tufto, Matt Kiersted, Adam Scheel, Walker Duehr).
It looks like Philadelphia signed Jackson Cates a day after the 2021 deadline. He played in some NHL games down the stretch but would have been ineligible for the playoffs had the Flyers qualified.
Boston signed Marc McLaughlin before the 2022 deadline, so he would have been playoff eligible but they sent him to the AHL playoffs after playing some NHL games. Handful of other guys signed but they were also sent to AHL (Corey Andonovski, Declan Carlile, Taylor Ward, Brandon Scanlin). Out of that bunch, Scanlin was the only one who got an ELC that started immediately; Everybody else signed an AHL ATO with an ELC that started 2022-23.