If you wish to make “shorter intervals for very mature card” available definitely, I would advise you to make a caveat to the users.
I noticed that, when using maximum interval (deck option), the add-on may have an undesired behavior. This is because it simply takes the original interval (that Anki would calculate without the add-on) and then applies the reduction factor. But as Anki already applies the Max Ivl when this interval is surpassed, the add-on will apply the reduction factor over an interval already shortened by Max Ivl, instead of calculating the interval considering the user configs and then, if this surpass Max Ivl, apply the reduction factor.
As an example, using these settings:
{ "days_lower": 450, "days_upper": 900, "delay: don't modify delay for cards with ivls below": 365, "delay: percentage": 75, "ease_max": 400, "ease_min": 140, "reduce_to": 50 }
And a Max Ivl of 2.5 years, a behavior like this would occur:
That is, if the user desire was to limit the Max Ivl to 2.5y, it wouldn’t work, and the interval would be reduced more than expected.
As a matter of fact, it is not a good idea to use this add-on with a MaxIvl set in the deck options.