Good news from the EASA! The following statement was sent by EASA to AOPA Germany on Aug 16 (published in their current newsletter):
Our below response addresses the case of non-large aircraft not used in commercial air transport.
We can confirm that the Cessna Supplemental Inspection Documents (SIDs) for 100/200 series are not included in the airworthiness limitations sections of the Cessna instructions for continuing airworthiness (ICA), and at this point they are also not covered by an AD. Hence, the Cessna SIDs for 100/200 series qualify as non-mandatory inspections in terms of ICA, even if they are designated "mandatory" in the revisions to the Cessna maintenance documentation. The position of the Agency is that compliance with SID for Cessna series aircraft should generally be recommended to aircraft owners/operators in line with the principles set out in M.A.302 and the related AMCs (cf. in particular Appendix I to AMC M.A.302 and AMC M.B.301(b) "Content of the Maintenance Programme", item 1.1.13a). If the owner/operator then decides not to include the optional modification/ inspections in the maintenance programme, he/she takes full responsibility for this decision.