All training can, and generally should be performed beltless, including maximal (one-repetition-maximum) lifts.
As I have discussed here at great length, there is no scientific evidence to support belted lifting, except for competition (performance) lifts, and only for those athletes who have always trained using a belt. The literature suggests that belts only assist us if we have developed our strength using them because they force us to adopt different lifting mechanics, and hence to adjust our posture and develop our structures to rely on them.
To summarise the key points from the post above:
- belts are unnecessary for elite-level strength development and performance;
- maximal lifts can be performed safely without a belt;
- activation of the transversus abdominis and internal oblique (or the ‘core’) is positively and significantly associated only with unbelted lifting;
- belts result in higher rates of spinal injury amongst elite powerlifters;
- belts have little or no effect on power, velocity, or range of motion; and
- belts offer little or no advantage to untrained individuals.
There is scant literature examining belted versus unbelted lifting in terms of elite performance, possibly due to the ubiquity of belts in lifting culture. But it should be understood that is just that—a culture. In my own experience as a strength and conditioning professional, I have not trained a single athlete using belts; nor have I worn a belt in my own training. And it is my experience that belts actually impair maximal lifting performance, probably due to altered lifting mechanics, consistent with the aforementioned theory. I have not witnessed a single injury in unbelted lifting, whether maximal (one-repetition maximum) or sub-maximal.
This does suggest, however, that if you are serious about developing core strength with unbelted training, it would be prudent to reduce your training loads temporarily whilst you focus on and adapt to the new lifting mechanics.
I hope that is helpful.