Samba 4 itself defaults to v3. That isn't per se a choice by SME.
There is nothing to stop anyone raising a bug and asking for a modfiable key, and possibly a v1 default.
you mean , nothing stop anyone from using existing modifiable key :
config setprop smb ServerMaxProtocol NT1
expand-template /etc/smb.conf
service smb restart
documented at least there:
https://wiki.contribs.org/Windows_10_Support#notes_about_Window_10_and_SME_Server_10 (the key exists also for SME9)
The question is what should be the default....
for fedora 26 The value default refers to the latest supported protocol, currently SMB3_11.
for SME 9 Default: max protocol = NT1 ; Default: min protocol = CORE (supported up to SMB2 ; but not the SMB2_10 and later version )
for SME 10 Default max protocol = SMB3 ; By default SMB3 selects the SMB3_11 variant; Default: server min protocol = LANMAN1
as we do not set any server max protocol ourself in the config (unless key is used) we then default our server to the default samba value.