Ashley, there are no contraindications to Thai Massage that are generally applicable. However it is important to ask the client a series of questions before the session so that you are well aware of possible issues. I have personally worked on highly pregnant women, people with cancer and those in remission, clients with Parkinsons and Multiple Sclerosis and many others.
You do have to adjust your sessions in some cases. That's why it is so important to have a wide repertoire of techniques ranging from strong to soft, and from stretching to rocking, plus adaptations for different body types and conditions.
But there is no contraindication that stipulates that you cannot do Thai Massage on a particular person with a particular condition. It is a matter of the skill and sensitivity of the practitioner.
Thai Massage can be very strong and intense or it can be featherlight and gentle. It does not have built in limitations aside from common sense scenarios that would apply to any type of massage.
However I should add that a fairly new or not very well trained practitioner should not attempt to work on people with conditions that require specific therapeutic skills.