Inactive subscribers

Subscriber count is cumulative. Someone who subscribed in 2019 and has not opened YouTube since is still in the total, which is how a channel gains subscribers every month while views fall. YouTube gives you no way to remove them and no report labelled inactive. The figures that describe your actual audience live elsewhere: returning viewers, unique viewers over the last 28 days, and the share of watch time coming from subscribers. Notifications thin themselves out anyway, since YouTube stops sending a channel's push alerts to anyone who has not watched or engaged with them for around a month, routing uploads to the inbox instead.