It is laughable to think microchips will be “required” to be implanted in humans at some point in the near future. Nevertheless, I chose to elaborate in a non condescending way as many people do have this question.
Microchips primary applications are for things we are concerned about losing and having a hard time identifying - like pets.
The main purpose for microchips are to monitor inventory and supply chain. They are also used for the purpose of reuniting missing pets with their owners. They rely on RFID- radio frequency identification data- an extremely reliable technology if microchips are scanned and properly registered. Currently- in the U.S. - when it comes to microchips and lost pets- there is no standardization. Multiple microchips companies, microchips, scanners, registries, and cataloging systems have made the overall potential of this technology ineffective. First off, there is no truly universal microchip scanner to date that can read the FULL range of microchips encountered by vets and shelters when they receive missing pets so pets are seldom recovered. If a microchip can be read- the number is displayed on the device. The number needs to be written down and multiple sites or registries need to be visited until a match is found. Then that registry needs to be called. It is extremely time consuming and veterinarians for example do not have the staff, time, or resources to accommodate for it.
More specifically, questions regarding humans, it's important to note that microchips are not GPS tracking devices and they are unnecessary for humans either way. Humans have unique fingerprints- whereas animals do not.
In regards to tracking- in time GPS will eventually and inevitably shrink down enough to be implanted, but at time of writing they are much too large. They will also need to rely on a power source. RFID tags (microchips) conversely are passive and activated when an RFID reader (microchip scanner) is waved over it. GPS will require a powering mechanism via kinetic energy. This is also much too large and will require years of testing to be safe.
Lastly- will it be a legal issue to NOT be chipped when the time comes?
It will first be a legal issue to chip as THAT would be a big brother act of abusive power, an obvious violation of privacy and the 4TH ammendment and it altogether wouldn't bode well as the general populace would contest it at the onset as they should. With that noted it will not be something to worry about anytime soon. Probationers and those on parole may be a different situation, but this will be something they will have agreed to as part of their conditions when it's set forth.