Discretization method as tag instead of enum
Tags (see https://git.iws.uni-stuttgart.de/dumux-repositories/dumux/-/blob/master/dumux/common/tag.hh) have some advantages over enums and, as far as I know, no disadvantages (edit: one disadvantage is that interfaces receiving a method as an argument need to be templated while this is not necessary for the enum implementation). The most prominent advantage being that a new tag can be added in user code without modifying the discretization/method.hh header which is currently required. This makes is difficult/impossible to add a new discretization in a separate module.
Tags have been introduced for the CouplingMode in couplingmanager1d3d.hh which previously was an enum and enabled implemented new modes in downstream modules. There was also a deprecation period possible which should be possible for the discretization method as well.