Good to hear that your FP5 was returned fixed.
There is quite a full discussion on the topic here https://www.reddit.com/r/LineageOS/comments/n7yo7u/a_discussion_about_bootloader_lockingunlocking/ and many contributions seen in a forum 
Search.
For others finding this thread it is unfortunate that documentation does not cover this point better.
To check the security patch level on your phone with a locked bootloader, prior to installing /e/OS, open your phone
Settings»About Phone»Android Version»Android Security Patch Level.Then compare it against the level of the security patch on the /e/OS build as visible in the Downloads for FP5 section below.
In the excitement of the phone arriving with Fairphone installed and starting the install adventure, I believe one must first go to
Settings » About Phone » Android Version » Android Security Patch Level
and take a screenshot of this information and store it off the device as internal storage will be wiped during the /e/OS install.
Following /e/OS install one can now compare that date with the date published here Releases · e / os / 🚀 Releases · GitLab and found when one searches that page for
Security fixes
where for instance one currently finds
This /e/OS v2.7 version includes the Android security patches available as of December 2024.