[RFC] nft - mss clamping (not) working?

kernel 4.14.131

with installed nft packages

kmod-nft-arp - 4.14.131-1.0
kmod-nft-bridge - 4.14.131-1.0
kmod-nft-core - 4.14.131-1.0
kmod-nft-fib - 4.14.131-1.0
kmod-nft-nat - 4.14.131-1.0
kmod-nft-nat6 - 4.14.131-1.0
kmod-nft-netdev - 4.14.131-1.0
kmod-nft-offload - 4.14.131-1.0
libnftnl - 1.1.2-1.0
libxtables-nft - 1.8.2-3.50
nft-qos - 1.0.6-1.35
nftables - 0.9.0-2.0

nf kernel settings
CONFIG_NETFILTER=y
CONFIG_NETFILTER_ADVANCED=y
CONFIG_BRIDGE_NETFILTER=m
CONFIG_NETFILTER_INGRESS=y
CONFIG_NETFILTER_NETLINK=m
CONFIG_NETFILTER_FAMILY_BRIDGE=y
CONFIG_NETFILTER_FAMILY_ARP=y
# CONFIG_NETFILTER_NETLINK_ACCT is not set
CONFIG_NETFILTER_NETLINK_QUEUE=m
CONFIG_NETFILTER_NETLINK_LOG=m
# CONFIG_NETFILTER_NETLINK_GLUE_CT is not set
CONFIG_NETFILTER_XTABLES=m
CONFIG_NETFILTER_XT_MARK=m
CONFIG_NETFILTER_XT_CONNMARK=m
CONFIG_NETFILTER_XT_SET=m
CONFIG_NETFILTER_XT_TARGET_CHECKSUM=m
CONFIG_NETFILTER_XT_TARGET_CLASSIFY=m
# CONFIG_NETFILTER_XT_TARGET_CONNMARK is not set
CONFIG_NETFILTER_XT_TARGET_CT=m
CONFIG_NETFILTER_XT_TARGET_DSCP=m
CONFIG_NETFILTER_XT_TARGET_HL=m
# CONFIG_NETFILTER_XT_TARGET_HMARK is not set
# CONFIG_NETFILTER_XT_TARGET_IDLETIMER is not set
CONFIG_NETFILTER_XT_TARGET_LED=m
CONFIG_NETFILTER_XT_TARGET_LOG=m
# CONFIG_NETFILTER_XT_TARGET_MARK is not set
CONFIG_NETFILTER_XT_NAT=m
CONFIG_NETFILTER_XT_TARGET_NETMAP=m
CONFIG_NETFILTER_XT_TARGET_NFLOG=m
CONFIG_NETFILTER_XT_TARGET_NFQUEUE=m
# CONFIG_NETFILTER_XT_TARGET_NOTRACK is not set
CONFIG_NETFILTER_XT_TARGET_FLOWOFFLOAD=m
# CONFIG_NETFILTER_XT_TARGET_RATEEST is not set
CONFIG_NETFILTER_XT_TARGET_REDIRECT=m
CONFIG_NETFILTER_XT_TARGET_TEE=m
CONFIG_NETFILTER_XT_TARGET_TPROXY=m
CONFIG_NETFILTER_XT_TARGET_TRACE=m
CONFIG_NETFILTER_XT_TARGET_TCPMSS=m
# CONFIG_NETFILTER_XT_TARGET_TCPOPTSTRIP is not set
CONFIG_NETFILTER_XT_MATCH_ADDRTYPE=m
CONFIG_NETFILTER_XT_MATCH_BPF=m
# CONFIG_NETFILTER_XT_MATCH_CGROUP is not set
CONFIG_NETFILTER_XT_MATCH_CLUSTER=m
CONFIG_NETFILTER_XT_MATCH_COMMENT=m
CONFIG_NETFILTER_XT_MATCH_CONNBYTES=m
CONFIG_NETFILTER_XT_MATCH_CONNLABEL=m
CONFIG_NETFILTER_XT_MATCH_CONNLIMIT=m
# CONFIG_NETFILTER_XT_MATCH_CONNMARK is not set
CONFIG_NETFILTER_XT_MATCH_CONNTRACK=m
# CONFIG_NETFILTER_XT_MATCH_CPU is not set
# CONFIG_NETFILTER_XT_MATCH_DCCP is not set
# CONFIG_NETFILTER_XT_MATCH_DEVGROUP is not set
CONFIG_NETFILTER_XT_MATCH_DSCP=m
CONFIG_NETFILTER_XT_MATCH_ECN=m
CONFIG_NETFILTER_XT_MATCH_ESP=m
CONFIG_NETFILTER_XT_MATCH_HASHLIMIT=m
CONFIG_NETFILTER_XT_MATCH_HELPER=m
CONFIG_NETFILTER_XT_MATCH_HL=m
# CONFIG_NETFILTER_XT_MATCH_IPCOMP is not set
CONFIG_NETFILTER_XT_MATCH_IPRANGE=m
CONFIG_NETFILTER_XT_MATCH_IPVS=m
# CONFIG_NETFILTER_XT_MATCH_L2TP is not set
CONFIG_NETFILTER_XT_MATCH_LENGTH=m
CONFIG_NETFILTER_XT_MATCH_LIMIT=m
CONFIG_NETFILTER_XT_MATCH_MAC=m
# CONFIG_NETFILTER_XT_MATCH_MARK is not set
CONFIG_NETFILTER_XT_MATCH_MULTIPORT=m
# CONFIG_NETFILTER_XT_MATCH_NFACCT is not set
# CONFIG_NETFILTER_XT_MATCH_OSF is not set
CONFIG_NETFILTER_XT_MATCH_OWNER=m
CONFIG_NETFILTER_XT_MATCH_POLICY=m
CONFIG_NETFILTER_XT_MATCH_PHYSDEV=m
CONFIG_NETFILTER_XT_MATCH_PKTTYPE=m
CONFIG_NETFILTER_XT_MATCH_QUOTA=m
# CONFIG_NETFILTER_XT_MATCH_RATEEST is not set
# CONFIG_NETFILTER_XT_MATCH_REALM is not set
CONFIG_NETFILTER_XT_MATCH_RECENT=m
# CONFIG_NETFILTER_XT_MATCH_SCTP is not set
CONFIG_NETFILTER_XT_MATCH_SOCKET=m
CONFIG_NETFILTER_XT_MATCH_STATE=m
CONFIG_NETFILTER_XT_MATCH_STATISTIC=m
CONFIG_NETFILTER_XT_MATCH_STRING=m
CONFIG_NETFILTER_XT_MATCH_TCPMSS=m
CONFIG_NETFILTER_XT_MATCH_TIME=m
CONFIG_NETFILTER_XT_MATCH_U32=m
nft kernel settings
CONFIG_NFT_EXTHDR=m
CONFIG_NFT_META=m
# CONFIG_NFT_RT is not set
CONFIG_NFT_NUMGEN=m
CONFIG_NFT_CT=m
CONFIG_NFT_FLOW_OFFLOAD=m
CONFIG_NFT_SET_RBTREE=m
CONFIG_NFT_SET_HASH=m
# CONFIG_NFT_SET_BITMAP is not set
CONFIG_NFT_COUNTER=m
CONFIG_NFT_LOG=m
CONFIG_NFT_LIMIT=m
CONFIG_NFT_MASQ=m
CONFIG_NFT_REDIR=m
CONFIG_NFT_NAT=m
# CONFIG_NFT_OBJREF is not set
# CONFIG_NFT_QUEUE is not set
CONFIG_NFT_QUOTA=m
CONFIG_NFT_REJECT=m
CONFIG_NFT_REJECT_INET=m
# CONFIG_NFT_COMPAT is not set
CONFIG_NFT_HASH=m
CONFIG_NFT_FIB=m
CONFIG_NFT_FIB_INET=m
CONFIG_NFT_DUP_NETDEV=m
CONFIG_NFT_FWD_NETDEV=m
# CONFIG_NFT_FIB_NETDEV is not set
CONFIG_NFT_CHAIN_ROUTE_IPV4=m
CONFIG_NFT_REJECT_IPV4=m
# CONFIG_NFT_DUP_IPV4 is not set
CONFIG_NFT_FIB_IPV4=m
CONFIG_NFT_CHAIN_NAT_IPV4=m
CONFIG_NFT_MASQ_IPV4=m
CONFIG_NFT_REDIR_IPV4=m
CONFIG_NFT_CHAIN_ROUTE_IPV6=m
CONFIG_NFT_CHAIN_NAT_IPV6=m
CONFIG_NFT_MASQ_IPV6=m
CONFIG_NFT_REDIR_IPV6=m
CONFIG_NFT_REJECT_IPV6=m
# CONFIG_NFT_DUP_IPV6 is not set
CONFIG_NFT_FIB_IPV6=m
CONFIG_NFT_BRIDGE_META=m
CONFIG_NFT_BRIDGE_REJECT=m
# CONFIG_NFTL is not set
# CONFIG_INFTL is not set

nft working peachy with one exception, that is TCP MSS clamping. According to source man

Since Linux kernel 4.14 and nftables 0.9 you can clamp your TCP MSS to Path MTU

Yet it produces

Error: Could not process rule: No such file or directory

which commonly implies some module or kernel setting missing.

For a router I would expect it to be working as advertised by source man.

Is this know however not to be working in OpenWrt?

Suspect that it caused by

# CONFIG_NFT_RT is not set