IF: How is it ensured within IF, that the employment modalities are conform with the requirements of MSCA?

The Declaration on the Conformity has indeed been removed from the list of contractual obligations of the H2020 Individual Fellowships just for simplification purposes.
This has not been considered appropriate in the case of ITNs, RISE and COFUND actions because, under these schemes, it is not yet known at the time of the Grant Agreement signature who will be recruited as a Marie Curie fellow. In the case of IFs, however, the fellow's name is already known at the proposal submission stage. Proposals must be linked to a specific applicant; evaluation is based, among other things, on the individual merits of the applicants and the fellowships are granted to these individual researchers. We won't accept the fellow being replaced by any other person during the development of the action.
Then, with regards to the conformity of the employment modalities, please note that by signing the Grant Agreement,  the host institution is obliged to respect some specific recruitment and working conditions (for more info please refer to Article 32.1 of the GA - Obligations towards the recruited researcher).