On my Linux Mint 19.1:

$ umask
0022

After the first run of bitcoin-qt -sysperm=false the 0700 permissions are expected for both the datadir (~/.bitcoin) and the walletdir (~/.bitcoin/wallets).

Actually, the directories are created with such access permissions:

$ stat ~/.bitcoin | grep Uid
Access: (0755/drwxr-xr-x)  Uid: ( 1000/ hebasto)   Gid: ( 1000/ hebasto)
$ stat ~/.bitcoin/wallets | grep Uid
Access: (0755/drwxr-xr-x)  Uid: ( 1000/ hebasto)   Gid: ( 1000/ hebasto)

Refs:

• Add option to disable 077 umask (create new files with system default umask) #4286
• -sysperms=false (default) doesn't appear to do anything #13371
• Utils and libraries: Fix #13371 - move umask operation earlier in AppInit() #13389

NOTE: reading through the initial PR #4286 makes me think it was controversial. Is it a better and simpler way to just get rid of -sysperm option? I believe a user who needs this option can easily run chown command.