Some sort of workaround seems to be implemented in SFOS 4.4.0.68.
The mobile data connection is still active, but at least the default route is deleted, so that a connection via IPv6 mobile data is no longer possible.
[defaultuser@Xperia10III ~]$ version
Sailfish OS 4.4.0.68 (Vanha Rauma)
[defaultuser@Xperia10III ~]$ ip -6 route
::1 dev lo proto kernel metric 256
2a01:598:7ff:0:10:74:210:210 via fe80::ccc6:701e:25e1:63fd dev rmnet_data1 metric 1 onlink
2a01:598:7ff:0:10:74:210:211 via fe80::ccc6:701e:25e1:63fd dev rmnet_data1 metric 1 onlink
2a01:598:a90d:af7e::/64 dev rmnet_data1 proto kernel metric 256
fe80::ccc6:701e:25e1:63fd dev rmnet_data1 metric 1 onlink
fe80::/64 dev rmnet_data0 proto kernel metric 256
fe80::/64 dev rmnet_data1 proto kernel metric 256
fe80::/64 dev wlan0 proto kernel metric 256
fe80::/64 dev rndis0 proto kernel metric 256
[defaultuser@Xperia10III ~]$ curl -6 ifconfig.net/
curl: (7) Couldn't connect to server
Not particularly elegant, but I’ll take that as a solution.