The session tick instrument currently includes a test subfield indicating whether the browser supports passive event listeners. As discussed on T261701, the purpose of this collection is to determine what percentage of browsers do not support passive event listeners and whether there exists a difference in the distribution of session length results between newer and older browsers (i.e., between browsers that support passive event listeners and those that do not). If a small enough percentage of browers do not support passive event listeners or if no substantial difference exists in the results between the two groups, we can stop collecting session length data from these older browsers in order to spare them the performance penalty of doing so.
See also
https://www.mediawiki.org/wiki/Compatibility#Browsers
https://caniuse.com/passive-event-listener