This interpretation was posted on 4/30/19:
18.104.22.168 A fielder shall not at any time obstruct a runner rounding a base.
22.214.171.124 A fielder shall not at any time obstruct a runner returning to or leading off a base.
The 126.96.36.199 rule in the 2020-2021 rule book has additional wording. The first sentence is the same as above. The new second sentence is: If a play is being made on the runner, the fielder may be charged with obstruction with no warning issued.
The option for issuing a warning is only for 188.8.131.52 (returning/leading off). For 184.108.40.206 (rounding), use the Effect – shall issue a warning. So, always issue a warning for rounding, whether a play is being made on a runner or not. For returning or leading off, issue a warning if no play is being made on that runner. You have an option to issue a warning or not if a play is being made on the runner.
A warning for obstruction on returning or leading off a base will be issued when the defensive player is not making a play on this runner and is merely in a position that obstructs the runner. If the defensive player is making a play on the runner, she will be charged with obstruction, and may be given a warning. Example: The runner at first base leads off and is returning to the base. The first baseman, in an attempt to get her out, obstructs the runner coming into first base. The defensive player is charged with obstruction, the runner is safe at first, but the warning is not required. The umpire still has the authority to issue the warning if he/she feel it is appropriate.