I am having this same error message problem. It has to do with a conflict between the JetPack plugin and WPtouch.
JetPack used to include an option for a JetPack-based mobile theme. However, the 8.3 and newer versions of Jetpack did away with that feature permanently. JetPack is no longer offering a mobile phone theme option.
SO, when I try to update my website to JetPack 8.3, it triggers this error message:
Notice: jetpack_check_mobile used for an anonymous function is deprecated since version with no alternative available. in /home/asahk7k15tad/public_html/wp-includes/functions.php on line 4651
This error message is triggered, apparently, because WPtouch hasn’t been updated, and so has some conflict with JetPack 8.3 and newer versions. (Could it also perhaps involve WordPress 5.3.2 and newer? Just asking.)
Anyway, please let me know what is being done with WPtouch to make it current and compatible with JetPack and also WordPress 5.3.2 and 5.4.
Thanks! I fully depend on the WPtouch plugin and hope it never goes away.
By the way, when I deactivate WPtouch, the error goes away. When I re-activate WPtouch, it comes back.