I strongly prefer sentence case.
Our style guide says to use sentence case for headings:
In the Python documentation, the use of sentence case in section titles is preferable, but consistency within a unit is more important than following this rule. If you add a section to a chapter where most sections are in title case, you can either convert all titles to sentence case or use the dominant style in the new section title.
https://devguide.python.org/documentation/style-guide/#capitalization
Readability
I find it more readable, and it's also an accessibility thing:
Use sentence case for headings (e.g. Don't auto-play video, music and more).
Don't write page titles in title case (with capital letters for every word). Use sentence case instead. Title case is more difficult for some people to read.
https://www.bath.ac.uk/guides/writing-accessible-web-page-titles-and-summaries/#writing-page-titles
Sentence case is easier to read.
People want to find out things quickly from your page, including whether they are on the right page. So headings need to be easy to scan and absorb.
Example
"Buying vegan cheese: what to consider" not "Buying Vegan Cheese What To Consider"
https://readabilityguidelines.co.uk/content-design/headings-titles/#4-use-sentence-case-for-headings-and-subheadings
Tech style guides
Sentence case
Google:
In document titles and headings, use sentence case. That is, capitalize only the first word in the title, the first word in a subheading after a colon, and any proper nouns or other terms that are always capitalized a certain way.
https://developers.google.com/style/capitalization#capitalization-in-titles-and-headings
Microsoft:
Use sentence-style capitalization for headings. That means that you capitalize the first word, any proper nouns, and the first word after a colon (if there is one). Everything else is lowercase. To learn more, see Capitalization.
https://learn.microsoft.com/en-us/style-guide/scannable-content/headings#formatting-headings
Title case
Red Hat:
The standard for technical documentation is title case for all headings and titles. Table titles and procedure titles fall into this category; standard title case capitalization rules apply.
https://stylepedia.net/style/#heading-styles
Both
Apple:
Except for user interface text, guidelines for when to use sentence-style capitalization and when to use title-style capitalization are a matter of department style.
https://support.apple.com/en-gb/guide/applestyleguide/apsgb744e4a3/1.0/web/1.0#apdca93e113f1d64