Generally, anything released starting April 25th, 2014—the date of the announcement of the new continuity model—should be considered Canon unless explicitly branded as Legends. Canon also includes the numbered Star Wars films and The Clone Wars:
While Lucasfilm always strived to keep the stories created for the EU consistent with our film and television content as well as internally consistent, Lucas always made it clear that he was not beholden to the EU. He set the films he created as the canon. This includes the six Star Wars episodes, and the many hours of content he developed and produced in Star Wars: The Clone Wars. These stories are the immovable objects of Star Wars history, the characters and events to which all other tales must align.
As you mention, Son of Dathomir is apparently part of both as well, since it's based on unproduced material for TCW and has apparently been published as Legends in some locales; Lucasfilm's Matt Martin appears to agree.
Definitely in both
As of time of writing, the following are all that's known to be part of both continuities:
- Star Wars Episodes I - VI
- Novelizations of Episodes I - VI where they don't contradict other stories
- The Clone Wars film and seasons 1-6
- Son of Dathomir
The film novelizations seem to have been largely ignored by newer canon, and the prequel novelizations in particular tend to contain a lot of references to Expanded Universe elements. I don't think Lucasfilm has explicitly said so, but from what I can tell, they only continue to be released as "Canon" to avoid making people think that the films themselves are no longer canon:
For example, the '76 Star Wars novelization calls Luke 'Blue Five' and has him do two trench runs. But if you were to slap a Legends label on it, you'd get people assuming all the events didn't happen. Not at all helpful.
[And if it helps put any of this into context; this is just my opinion. I am not (and have never been) "in charge of canon" so this means nothing as far as how things were done, are done, or will continue to be done]
(2021 tweets by Pablo Hidalgo)
The Fantasy Flight RPG sourcebooks have been published both before and after the Canon reset, and it isn't currently clear which (if any) continuity they're part of. Wookieepedia, for example, seems to have a best-guess policy depending on the publication date of the source and the origin of individual elements that have appeared in stories from one continuity but not the other.
Blade Squadron was technically published a few days before the Canon announcement (April 22nd, 2014), but was announced as being within the new Canon:
Fiction in Insider
Starting with the two-part Blade Squadron story (published in #149 and #150), fiction in Star Wars Insider is a part of the new canon.
(Star Wars Insider #150 via Lightsaber Rattling)
There are other Canon works that started life as Expanded Universe projects (such as Tarkin), or are otherwise compatible with details from Legends, but they're still considered part of the Canon continuity due to their release timing and branding.
Other works were released well after the Canon change, but were still released as Legends at time of first publication. For example, Issue 108 of the 1977 Marvel Star Wars comic series was released in 2019, five years after the new Canon announcement and 33 years after the previous issue, but is still branded as Legends.
The Old Republic has also continued to receive new content, despite being firmly set in the Legends continuity. However, in recent years, some of it has been explicitly inspired by Canon material, such as The Mandalorian:
Inspired by The Mandalorian, check out the newest Armor Sets, Weapons, mounts, and more, available now on the Cartel Market!
Cartel Market Additions: Game Update 6.1.4 on swtor.com
This isn't meant to imply that SWTOR takes place in Canon, however, and it's unclear to what degree in-game items are considered canonical anyway.