The rules do not seem to protect the area under your opponent's platform. Objects under the platform can deny your opponent the ability to drive a robot onto their platform, and they can prevent a platform from being balanced. The rules do not seem to prevent this defensive strategy.
The Q&A is closed for the 2021-2022 season. Any rule changes or clarifications pertaining to the 2022 VEX Robotics World Championship will be included in the April 5 Game Manual Update. Teams attending VEX Worlds who wish to pre-submit questions for the driver's meeting should have received a contact form via email; if not, please contact your REC Foundation Team Engagement Manager for more information.
Is it allowable to push Game Objects under your opponent's platform?
Edit 2021-12-07: The v2.2 Game Manual update included the following revision to rule <SG3>, which we believe answers this question more thoroughly than the original answer. We recommend reviewing the full text of <SG3>, including the “red box” clarifications, for more information.
Point “d” applies to Robot actions prior to the last thirty (30) seconds of the Match:
d. Placing a Scoring Object on or under the opposing Alliance’s Platform, at any point during the Match is considered a minor violation of this rule that, at a minimum, will result in a warning.
i. If the placement is accidental, and immediately rectified (i.e. the Scoring Object is immediately removed), then this violation will be considered a warning.
ii. If the placement is intentional and / or not immediately rectified, as judged by the Head Referee, then it will be considered a violation.
iii. Repeated, strategic, and / or egregious warnings may also escalate to a violation, at the Head Referee’s discretion.
Violations of this rule which do interfere with gameplay will result in a Disqualification, regardless of whether the interference was Match Affecting or not.
Note: If point “d” has escalated from a warning into a violation, then it will automatically be considered a violation which has interfered with gameplay, i.e. will result in a Disqualification.