According to the manual page of journalctl(1), it should accept the "--output"
options (e.g. "short-unix", or "json") as defined in the manual. However, it
seems that with the "--list-boots" the "--output=<whatever>" is ignored.
Example:

 # journalctl --output=short-unix --list-boots
 -3 bfdc1672ba224e4a84aa4655ed90ec0a Thu 2023-02-23 02:11:17 CET���������Fri 2023-02-24
06:13:16 CET
 -2 26ef56c47e734985b3805a8af7f394c3 Fri 2023-02-24 06:20:50 CET���������Sat 2023-02-25
08:29:24 CET
 -1 6132b2a1bc4c4ac1b0bc0c4afc9bda5a Sat 2023-02-25 08:30:17 CET���������Sat 2023-02-25
14:50:14 CET
  0 666e5a91b4274dac87640d1199dac62c Sat 2023-02-25 14:50:37 CET���������Sun 2023-02-26
11:01:50 CET

 # journalctl --output=json --list-boots
 -3 bfdc1672ba224e4a84aa4655ed90ec0a Thu 2023-02-23 02:11:17 CET���������Fri 2023-02-24
06:13:16 CET
 -2 26ef56c47e734985b3805a8af7f394c3 Fri 2023-02-24 06:20:50 CET���������Sat 2023-02-25
08:29:24 CET
 -1 6132b2a1bc4c4ac1b0bc0c4afc9bda5a Sat 2023-02-25 08:30:17 CET���������Sat 2023-02-25
14:50:14 CET
  0 666e5a91b4274dac87640d1199dac62c Sat 2023-02-25 14:50:37 CET���������Sun 2023-02-26
11:02:02 CET

For scripting it would be valuable to have the output formatted as described in
the manual page.