I think it is an oversight. The txt-Format was included up to MATSim-0.7.0 for backwards compatibility with older visualization software, dating back to the first versions of MATSim, which are now about 10 years old. But the txt-Format wasn't used anymore for a few years, and so support for it was removed in MATSim-0.8.0. Looks like the description of the parameter was not adapted to this change.
In short: txt is no longer supported, only xml is supported for Events in MATSim-0.8.0.