As the line you quote says, the details are in the source file. It is distributed in LPPL with an additional clause:
% \iffalse meta-comment
% This file is part of the Standard LaTeX `Tools Bundle'.
% It may be distributed and/or modified under the
% conditions of the LaTeX Project Public License, either version 1.3c
% of this license or (at your option) any later version.
% The latest version of this license is in
% and version 1.3c or later is part of all distributions of LaTeX
% version 2005/12/01 or later.
% The list of all files belonging to the LaTeX `Tools Bundle' is
% given in the file `manifest.txt'.
% \iffalse This is a METACOMMENT
%% Package `multicol' to use with LaTeX2e
%% Copyright 1989-2018 Frank Mittelbach
%% In addition to the terms of LPPL any distributed version
%% (unchanged or modified) of multicol has to keep the statement
%% about the moral obligation for using multicol. In case of major
%% changes where this would not be appropriate the author of the
%% changed version should contact the copyright holder.
%% Moral obligation for using multicol:
%% Users of multicol who wish to include or use multicol or a modified
%% version in a proprietary and commercially market product are asked
%% under certain conditions (see below) for the payment of a license
%% fee. The size of this fee is to be determined, in each instance,
%% by the commercial user, depending on his/her judgment of the value of
%% multicol for his/her product.
%% The conditions for this are as follows:
%% The producer of a proprietary and commercially market product
%% that involves typesetting using multicol is asked to determine
%% the value of a license fee for using multicol if
%% - the product is a document and the producer has decided to
%% include multicol to typeset (parts of) the document or has
%% directed the author of the document to include multicol (for
%% example, by providing a class file to be used by the author)
%% - the product is a LaTeX class or package that includes multicol
%% There is no moral obligation in case
%% - the product is a document but producer has not directed
%% the author to include multicol (in that case the moral obligation
%% lies with the author of the document)
%% - the product does not involve typesetting, e.g., consists, for
%% example, of distributing multicol and its documentation.
%% - the product is not proprietary, i.e., is made available as free
%% software itself (which doesn't prohibit its commercial marketing)
%% - multicol is used for non-commercial purposes
%% Determining a license fee might result in a license fee of zero
%% (i.e., no payment) in case a producer has determined that the use
%% of multicol has no enhancing effect on the product. This is a
%% plausible scenario, i.e., in the above two cases the producer is
%% only asked to evaluate the value of multicol for the product
%% not for the payment of a license fee per se (which might or might
%% not follow from this evaluation).
%% The license fee, if any, can be payed either to the LaTeX3 fund
%% (see ltx3info.txt in the base LaTeX distribution) or to the author of
%% the program who can be contacted at