Synology models with PCIe slots but don't (officially) support NVMe drives.
DS3611xs
DS3612xs
DS3615xs
FS2500
FS3017 2 PCIe slots
FS3600
FS6400 2 PCIe slots
RS18016xs+
RS3411RPxs
RS3411xs
RS3412RPxs
RS3412xs
RS3413xs+
RS3614RPxs
RS3614xs
RS3614xs+
RS3617xs 2 PCIe slots
SA3200