Looking at the data collected yesterday, 2/8/21, on the group 0 and 1 wikis:

• Only 20,569 (0.77%) out of 2,656,586 initial ticks (tick = 0) were from browsers that did not support passive event listeners.
• That said, the initial dropoff between ticks 0 and 1 was much sharper for the group that did not support passive event listeners than for the population as a whole. Only 10,138 (49.29%) of sessions in browsers not supporting passive event listeners made it to tick 1, vs. 1,993,578 (75.04%) of all sessions. After tick 1, the dropoff rate between subsequent ticks appears roughly similar.

Also related: https://phabricator.wikimedia.org/T271168#6784257

Change 663291 had a related patch set uploaded (by Jason Linehan; owner: Jason Linehan):
[mediawiki/extensions/WikimediaEvents@master] Disable sessionTick on browsers w/o passive event listener support

https://gerrit.wikimedia.org/r/663291

Change 663291 merged by jenkins-bot:
[mediawiki/extensions/WikimediaEvents@master] Disable sessionTick on browsers w/o passive event listener support

https://gerrit.wikimedia.org/r/663291

Thanks @Mholloway ! Some followups:

The data collected showed that fewer than 5% of clients lack passive event listener support

On T274264#6816810, you wrote 0.77% ; would it be reasonable to say "fewer than 1%"?

and a cursory examination showed no meaningful difference in their distributions.

Suggested revision (again based on your earlier comment):

Browsers that don't support passive event listeners are less likely to progress beyond tick 0 (50% continue past tick 0, compared to 75% for all sessions). After tick 1, the drop-off rate between subsequent ticks appears roughly similar to all browsers.

(I think that's a meaningful difference, although we decided at this time that collecting the data from old browsers is not worth risking a negative user experience based.)

In T274264#6873216, @kzimmerman wrote:

Thanks @Mholloway ! Some followups:

The data collected showed that fewer than 5% of clients lack passive event listener support

On T274264#6816810, you wrote 0.77% ; would it be reasonable to say "fewer than 1%"?

Yes, better.

and a cursory examination showed no meaningful difference in their distributions.

Suggested revision (again based on your earlier comment):

Browsers that don't support passive event listeners are less likely to progress beyond tick 0 (50% continue past tick 0, compared to 75% for all sessions). After tick 1, the drop-off rate between subsequent ticks appears roughly similar to all browsers.

(I think that's a meaningful difference, although we decided at this time that collecting the data from old browsers is not worth risking a negative user experience based.)

Yep, makes sense to me!

@kzimmerman Updated the wiki; please let me know if that looks good!