The good news is that the family sharing feature is now being extended to third-party apps as well. This feature was actually announced during WWDC 2020 where Apple engineer Tori Sherman was quoted as saying, “We’re very excited to announce that this year the app store will support Family Sharing for in-app purchases, which will allow up to five additional family members to share a single purchase.”
However, it should be noted that whether or not a third-party app chooses to allow you to share an in-app purchase or subscription is entirely up to the developers themselves. This means that not all apps will allow sharing, and not all developers will participate, but the option is there for them to take advantage of it if they choose.
In some cases, it might benefit the developer to allow sharing as it could encourage users to purchase a subscription where they might not have otherwise done so, but this will vary from app to app, so whether or not your apps will support the feature is a different story.